This invention relates in general to optical switches and more particularly, to polarization-independent optical switches suitable for use with fiber optical transmission lines.
Optical directional couplers, such as those formed by two parallel channel waveguides, are characterized by (1) the interaction length L, (2) the coupling coefficient K or the corresponding conversion length l=.pi./2k ndicating the minimum length required to obtain complete crossover of light from one guide to the other, and (3) the mismatch .DELTA..beta.=.beta..sub.1 -.beta..sub.2 between the propagation constants .beta..sub.1 and .beta..sub.2 of the two guides. Complete crossover is achieved when the guides are phase matched (.DELTA..beta.=0) and when the interaction length is an exact odd multiple of the coupling length, i.e., when L=(2.upsilon.+1)l. An optical switch can be built by electrically switching the directional coupler from the crossover state to the straight-through state where no net crossover occurs. One way to do this is by fabricating the coupler on electrooptic material and applying a voltage which induces a mismatch .DELTA..beta. via the electrooptic effect. When the interaction length L is not made exactly equal to the coupling length l (or an odd multiple thereof), the the crossover is not complete and crosstalk results. In addition, the length l is a function of wavelength.
In reversed .DELTA..beta. couplers the technique used to achieve complete crossover in the coupler is to provide along the interaction length two or more sections with a mismatch or asynchronism .DELTA..beta. of alternating sign. A simple way to induce this alternating .DELTA..beta. is to provide sectioned electrodes and apply voltages of alternating polarity along the interaction length. There is no requirement for an exact L/l ratio in this configuration, and there is always a voltage that will make the light cross over completely, and another voltage that will make the light go straight through. If the switch has to be operated at another wavelength and l is wavelength dependent, the only adjustment that seems necessary, for small wavelength changes, is a change of these voltage values.
The polarization properties of optical switches are of great importance in determining the usefulness of these devices in an optical data transfer system employing fiber transmission lines. In particular, these devices must perform efficient and complete switching of light, without regard to its state of polarization. This requirement arises because linearly polarized light coupled into single mode, circular fibers suffers a rapid conversion to other polarization states. Light coupled from a fiber is therefore expected to possess an unknown elliptical polarization and both TE- and TM-like modes will be excited in the optical circuit. Any optical switch must act in identical fashion upon each of the constituent polarizations in order to achieve suitably low interchannel crosstalk.
Alferness in Appl. Phys. Lett. 35, 748 (1979) discloses a 2.times.2 optical switch that, for fixed switching voltages, operates with low crosstalk independent of the polarization of the input optical signal.
The difficulty of achieving efficient switching (i,e., low channel crosstalk) for both TE and TM polarizations with the same applied voltage arises because the orthogonal modes see unequal electro-optic coefficients. As a result, for the same applied voltage, the induced phase mismatch is different for the two polarizations. In addition, because the guide-substrate refractive index difference is generally unequal for the TE and TM modes, the mode confinement and consequently the coupling coefficient K depend upon polarization. The values of K and .DELTA..beta. together with the interaction length L determine the switching efficiency, and therefore the polarization dependence.
The device described in the Alferness reference is a reversed-.DELTA..beta. switch with weighted coupling, i.e., varying interguide separation, designed so as to allow polarization-independent behavior. Reversed-.DELTA..beta. type electrodes are formed over the waveguides so that voltages can be applied to achieve the required switching conditions by means of the electro-optic effect. The design requires the measurement of the coupling characteristics of singlemode channel waveguides in order to determine, for each optical polarization, the coupling coefficient as a function of coupler spacing. The coupling characteristics are dependent on the channel waveguide fabrication parameters such as the waveguide width, the thickness of the metal diffused into the electrooptic substrate to create a waveguiding region of higher refractive index than the substrate and the diffusion time, temperature and gas flow conditions. The relationship between coupling coefficient K and interguide separation d can be described approximately by the relationship for coupling between two planar waveguides: EQU K=K.sub.o exp (-d/.gamma.)
where .gamma. is the waveguide transverse penetration depth. Both K.sub.o and .gamma. depend on the waveguide fabrication parameters. The Alferness reference states that K.sub.TE &gt;K.sub.TM for large d (d&gt;d.sub.e) and K.sub.TE &lt;K.sub.TM for small d(d&lt;d.sub.e), where the subscript TE refers to the TE mode polarization and TM refers to the TM mode polarization. The interguide spacing d.sub.e at which K.sub.TE and K.sub.TM intersect is determined from experimental measurements. The variable-spacing switch is then designed about this intersection point and the waveguide fabrication conditions which resulted in d=d.sub.e for K.sub.TE =K.sub.TM must be replicated when the switch is fabricated. The waveguide spacing in the switch is determined by d.sub.e and, since the electrodes lie on top of the waveguides, the electrode spacing g is also dependent on d.sub.e. Hence, the switching voltage is determined by d.sub.e. The electrode spacing cannot be varied in order to optimize the switching voltage but is restricted to a value dependent on the different variations of K.sub. TE and K.sub.TM with d.